The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated device, such as a steering cylinder.
Fluid controllers of the type to which the present invention applies are typically used to control the flow of fluid to the steering cylinder of vehicles such as agricultural tractors. Many such tractors are used as row crop tractors, which involves driving the tractor along the entire length of a straight row of a particular crop, while performing some agricultural function, such as harvesting.
For as long as such controllers have been utilized in steering systems of such tractors, it has been noticed by the vehicle operator that in order to steer a straight path along the row, the neutral position of the steering wheel does not remain constant, but gradually "precesses", i.e., it rotates in one direction or the other from the original centered position. Such steering wheel precession is considered by the vehicle manufacturer to be a very undesirable characteristic of the steering system, and particularly of the controller, partly because the tendency is for the vehicle operator to believe that there is a malfunction of the steering system.
The assignee of the present invention has provided one partial solution to the problem of steering wheel precession by developing and providing commercially a fluid controller in which the valving and the various ports and passages therein are substantially symmetrical about a central reference plane. See U.S. Pat. Nos. 4,037,620 and 4,212,229, both of which discuss the problem of steering wheel precession. However, the symmetrical valving design disclosed in the above cited patents is in use commercially in only the relatively larger and more expensive fluid controllers (i.e., those in the 20-50 gpm range). Typical agricultural tractors which are driven along a straight row of crops as described above typically use relatively smaller, less expensive fluid controllers (i.e., in the 3-15 gpm range), and such controllers continue to utilize the more conventional, non-symmetrical type of valving.
In producing controllers of the type to which this invention relates, it has for many years been an objective of the assignee of the present invention to carefully tune or balance the timing or phasing of the key flow control orifices, which are the main variable flow control orifice for both right turn and left turn conditions (hereinafter, A1-R and A1-L) and the return flow control orifice for both right turn and left turn conditions (hereinafter, A5-R and A5-L). This tuning or balancing has been accomplished for many years by means of an air pressure test, during the assembly of the controller, the purpose of which has been to insure that all four of the above-noted orifices begin to open at substantially the same timing (i.e., in terms of degrees of relative rotation between the valve spool and valve sleeve). If a discrepancy was noted in the timing, a correction would be made. For example, if the timing of the A1-L orifice lagged the others by a fraction of a degree, there would be additional machining (grinding) performed on the wall of one of the axial slots comprising that orifice, as a result of which its timing would be advanced to be substantially equal to that of the other orifices. Until the time of the present invention, it has been believed that one important reason for tuning or balancing the timing of these orifices was to keep the problem of steering wheel precession from becoming worse than it already was.